The natural killer (NK) activity of an allogeneic tumor system was found to be enhanced by the total splenic population of either pregnant of normal female BALB/c mice treated with human chorionic gonadotropin (HCG) over those values detected for the nonpregnant, untreated controls. The splenic population of mice in the early stage of pregnancy (9-10 days) exhibited a statistically significant increase in NK activity over nonpregnant controls at all effector/target ratios tested. The groups of mice in the mid (12-13 days) and late (15-16 days) stages of pregnancy also showed significantly enhanced NK activity above control values at effector/target ratios of 100:1, 50:1 and 25:1. The i.v. administration of carrageenan into pregnant mice abolished the increased NK activity suggesting a putative role for the macrophage in this system. HCG administered to normal female BALB/c mice resulted in enhanced NK activity similar to that seen for the pregnant mice. This augmented NK activity does not appear to be effected by either the number or timing of the HCG administration. The augmentation of NK activity of HCG-treated mice was also detected in a syngeneic system utilizing 51Cr labeled M109 tumor target cells and BALB/c effector cells.